Internal combustion engines may produce considerable quantities of nitrogen oxides (NOx) during operation. In particular in diesel and Otto engines used in motor vehicles, the nitrogen oxide quantities in the exhaust gas are generally above the permissible limit values so that an exhaust gas reprocessing is demanded in order to reduce the NOx emissions. In many engines, the reduction of the nitrogen oxides is carried out by means of the non-oxidized components contained in the exhaust gas, that is to say, by carbon monoxide (CO) and non-combusted hydrocarbons (HC), using a three-way catalytic converter. In particular with diesel and Otto lean engines, as a result of the low quantities of non-oxidized exhaust gas components, however, this method is not available. In lean engines, therefore, in accordance with a widespread method there is used a NOx storage catalytic converter (referred to in abbreviated form below as LNT, Lean NOx Trap), which absorbs and stores the nitrogen oxides contained in the exhaust gas of the internal combustion engine. From time to time, a regeneration of the LNT is carried out, for which, for example, an excess of fuel is produced in the exhaust gas directed through the LNT.
However, the inventors herein have recognized potential issues with such systems. As one example, HP-EGR is activated during transient operations to meet driver demand, which may result in ammonia slip from a catalyst. While ammonia is slipping from the catalyst, LP-EGR may not flow, which may result in increased engine NOx output.
In one example, the issues described above may be addressed by a method comprising activating an electric motor and flowing low-pressure exhaust gas recirculation (LP-EGR) flow for a threshold duration in response to an ammonia slip risk of a catalyst being present, and deactivating the electric motor, shutting off LP-EGR flow, and flowing HP-EGR in response to the ammonia slip risk being present after the threshold duration. In this way, ammonia slip from the catalyst is decreased and/or prevented for at least the threshold duration of the ammonia slip risk.
As one example, the ammonia slip risk is based on an increase in driver demand, where the increase in driver demand results in engine conditions promoting desorption of ammonia stored on the catalyst. For example, the increase in driver demand may result in higher exhaust gas temperatures which may induce desorption of ammonia from the catalyst. Due to the catalyst being arranged upstream of an LP-EGR passage, desorbed ammonia may be swept to the engine when flowing LP-EGR. Thus, to prevent ammonia slip from the catalyst and limit engine NOx output, engine speed and torque may be reduced and/or maintained at current levels in response to the increase in driver demand. As such, exhaust gas conditions may remain substantially similar to exhaust gas conditions prior to the increase in driver demand.
The electric motor may remain active for the threshold duration. In one example, the threshold duration is based on a current battery state-of-charge (SOC). Thus, the battery may run out of energy once the threshold duration is complete and no longer be able to power the electric motor. In response to completion of the threshold duration, the electric motor is deactivated, and engine speed and torque are increased. Additionally, HP-EGR flows to the engine and LP-EGR is shut off to prevent ammonia from flow to the engine. In some embodiments, the catalyst may be a first selective reduction catalyst (SCR), wherein there may be arranged a second SCR downstream of the LP-EGR passage. The second SCR may capture some of the desorbed ammonia from the first SCR when an ammonia store of the second SCR is less than threshold fill store (e.g., 100% saturation of ammonia).
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.